The invention concerns an arrangement for adjusting the spatial dependence of a magnetic field in a working volume of a main field magnet using ferromagnetic field shaping elements.
An arrangement of this type is known from U.S. Pat. No. 4,990,877.
For magnetic resonance apparatus, an approximately spherical working volume is usually desired and realized in which the magnetic field should be as homogeneous as possible, i.e. the desired spatial dependence of the magnetic field should be a constant. For modern magnetic resonance apparatus which are used in medical diagnosis, the maximum admissible relative deviation of the magnetic field from its average value is typically less than 5 ppm (parts per million) in a working volume with a diameter of approximately half the diameter of the bore of the main field magnet. Initially, the precondition for such a precisely required spatial dependence of the magnetic field is a suitable geometric arrangement of the components of the main field magnet which produce the magnetic field, which can be theoretically calculated. However, in practice there are deviations from the ideal theoretical geometric arrangement of the components of the main field magnet due to mechanical tolerances during production of the main field magnet which can produce actual deviations from the desired constant magnetic field on the order of 1000 ppm.
All conventional embodiments for adjusting the spatial dependence of magnetic fields using ferromagnetic field shaping elements (also designated passive shim systems or homogenization means) use a plurality, typically more than one hundred, predetermined locations at which one or more ferromagnetic field shaping elements are mounted. The field shaping elements can e.g. be square sheet metal pieces which can be easily stacked on top of each other to be mounted at one of the predetermined locations. Particularly inexpensive types of field shaping elements are standard parts such as washers or nuts or screws made from magnetic steel. An unavoidable feature of these systems with a plurality of predetermined locations for the shims is that each of the predetermined locations must have devices for holding the ferromagnetic field shaping elements at the respective location, despite the presence of large magnetic forces. For this reason, all known means for adjusting the spatial dependence of magnetic fields using ferromagnetic field shaping elements have the disadvantage that a relatively large amount of volume is lost due to the retaining devices themselves and there are gaps between the field shaping elements which cannot be utilized. In order to reduce production costs, magnets for producing the highest magnetic fields (above 10T) for magnetic resonance spectroscopy have a tubular access to the working volume with a diameter of only a few cm. In such cases, conventional means for adjusting the spatial dependence of magnetic fields using ferromagnetic field shaping elements cannot be used since the space required is excessively large. A further disadvantage of these conventional means is that loading must be carried out manually and counting of the required field shaping elements is required for each of the numerous predetermined locations. Occupation errors are likely, the correction of which is time consuming. In U.S. Pat. No. 4,990,877, the main field magnet is part of a magnetic resonance apparatus and has a cylindrical bore for receiving a patient. The working volume of the main field magnet is thereby located in the center of the bore. A plurality of field shaping elements made from ferromagnetic material is mounted to the surface of the bore of the main field magnet. These field shaping elements can consist of soft-magnetic material, e.g. soft iron and are magnetized to a calculatable value, e.g. saturation magnetization, in the magnetic field of the main field magnet for influencing the spatial dependence of the magnetic field in the working volume. Realization of a suitable geometric arrangement of the field shaping elements permits adjustment to the desired spatially constant magnetic field. This process is usually carried out in three steps.
In an initial measuring step, the actual spatial dependence of the magnetic field in the working volume is measured. The geometric arrangement of the field shaping elements which is required for adjusting the desired spatial dependence is then calculated in a calculating step. Finally, this calculated geometric arrangement of the field shaping elements is realized in an “occupation step”. The improved spatial dependence of the magnetic field can be iteratively optimized in further measuring, calculating and occupying steps.
The occupying step requires a suitable means for mounting the field shaping elements. Such means must meet several requirements. Since large magnetic forces act on the ferromagnetic field shaping elements in the background field of the main field magnet, a correspondingly stable anchoring of the field shaping elements must be ensured. Moreover, the position of the field shaping elements must be precisely fixed and reproducible in case of iterative optimizations. It is also important that the means requires as little space as possible since the space in the bore for additional components such as e.g. gradient coil systems and the patient would otherwise be inadmissibly reduced or a correspondingly larger and therefore considerably more expensive main field magnet would be required. In addition, handling of the means should be simple.
In view of these features of prior art, it is the object of the present invention to facilitate precise adjustment of the spatial dependence of the magnetic field in the working volume of a main field magnet using ferromagnetic field shaping elements to optimally reduce the volume lost through holding devices for the field shaping elements while preventing occupation errors.